Estimating root respiration, microbial respiration in the rhizosphere, and root-free soil respiration in forest soils

We hypothesized that respiration measurements made using both the basal and excised-root respiration methods would allow us to quantify three important components of soil respiration: root respiration (Rroot), microbial respiration in the rhizosphere (Rrhizo), and root-free soil respiration (Rrfs). Root respiration determined by the basal method was approximately one-third greater than root respiration determined by the excised-root method (52 versus 32% of total soil respiration, respectively). Results from a decomposition model constructed for the root C fractions (easily decomposable, slowly decomposable, and recalcitrant) showed that the easily and slowly decomposable C fractions disappeared approximately 3 months after the basal respiration measurements began. Since these C fractions contained the majority of the C source for rhizosphere microorganisms (i.e., rhizodeposition), microbial respiration in the rhizosphere must have been severely reduced, indicating that respiration measured after 3 months must be dominated by microorganisms in the root-free soil (Rrfs); this indicated that the basal method actually measured root-free soil respiration. This allowed us to fractionate the components of soil respiration based on the relationship: total soil respiration (Rtotal)=Rroot+Rrhizo+Rrfs; subtracting Rroot(excised-root method) and Rrfs(basal method) from Rtotal gave us an estimate of microbial respiration in the rhizosphere; and the contribution of these three components to Rtotal: Rroot=32%, Rrhizo=20%, and Rrfs=48%. These results are important because they suggest a way that soil respiration can be separated into at least three functionally different components, and they show that microbial respiration in the rhizosphere is a significant sink for photosynthetically-fixed C in forests.